Yes, consolidation is definitely one of the major goals for virtualization. In my previous post I should have said that once you have your environment virtualized and everything is replicated that needs to be replicated, shut the other physical servers down.
You can absolutely have two independent ESX hosts, they will operate fine; however, you will need to manage them separately. Most shops will have at least two hosts using shared storage (aka SAN) so that you can leverage the built in high availability/DRS options in VMWare. With the HA set up you can tolerate a complete host failure, such as the mother board situation you are talking about, and VMWare will (depending on your settings) transfer (VMotion) the virtual machines over to the other host with minimal down time (minutes, not hours). You can also leverage the DRS capabilities so that, say you have three VM's on one host and 3 VM's on the other host. If one of your VM's is going crazy (like Exchange can do) and consuming a lot of resources on the host, VMWare can automatically move the other VM's on the host to the other one with zero downtime. The key to the whole HA/DRS configuration is utilizing a SAN.
With all of the VM's you want to run on a single host with the RAID 5, I am thinking that your disk I/O is going to be nuts especially with Exchange running so performance may (read: will) be an issue.
For backups, you can use things like VMWare Consolidated Backup or simply purchase BackupExec, CommVault, etc. These products can backup the VM's and the .vmdk files.
After all is said and done, this will be an expensive proposition up front. That being said, it is sooooo worth it in the long run. And yes, VMWare is what I would recommend. Sorry for the long post.
I hate all Uppercase... I don't want my groups to seem angry at me all the time! =)
- ColdFlame (vbscript forum)